A wide variety of mental and physical processes are controlled or influenced by neural activity in particular regions of the brain. For example, various physical or cognitive functions are directed or affected by neural activity within the sensory or motor cortices. Across most individuals, particular areas of the brain appear to have distinct functions. In the majority of people, for example, the areas of the occipital lobes relate to vision; the regions of the left interior frontal lobes relate to language; portions of the cerebral cortex appear to be consistently involved with conscious awareness, memory, and intellect; and particular regions of the cerebral cortex as well as the basal ganglia, the thalamus, and the motor cortex cooperatively interact to facilitate motor function control.
Essential tremor is a frequently occurring, complex neurologic movement disorder. At this time, the causes of essential tremor are not well understood.
Essential tremor (or ET) typically affects the hands, but it can also affect the head and neck (causing shaking), the face, jaw, tongue, voice, trunk, and, on occasion, the legs and feet. The tremor can take the form of a rhythmic lateral motion or forward and aft motion produced by involuntary muscle contractions. The duration and intensity of the tremors can vary substantially from one day to the next and during the course of a given day. ET typically has two forms: postural tremor, which occurs when the patient holds the affected muscle in a particular position, and kinetic tremor, which occurs when the patient moves the affected muscle in a particular way. Most patients affected by ET have both postural and kinetic tremor symptoms.
Effectively treating ET can be very difficult. Current treatments for ET symptoms include drugs, surgical intervention, and/or neural stimulation. Drug treatments or therapies may involve the administration of a beta-adrenergic blocker or anticonvulsant medication to the patient. Drug therapies may involve propanolol, mysoline, primidone, benzodiazepine, or a weak solution of botulinum toxin A. Unfortunately, many patients cannot tolerate or fail to adequately respond to drug therapies.
Surgical intervention for ET typically includes a thalamotomy, a procedure that involves ablating or destroying a selected portion of the thalamus. Unfortunately, surgical intervention is a very time consuming and highly invasive procedure. Potential complications associated with the procedure include risk of hemorrhage, stroke, and/or paralysis. Furthermore, because the procedures permanently destroy neural tissue, the effects of such intervention cannot be readily adjusted or “fine tuned” over time.
Neural stimulation treatments have shown promising results for reducing some of the symptoms associated with ET. Neural activity is governed by electrical impulses or “action potentials” generated in and propagated by neurons. While in a quiescent state, a neuron is negatively polarized and exhibits a resting membrane potential that is typically between −70 and −60 mV. Through chemical connections known as synapses, any given neuron receives excitatory and inhibitory input signals or stimuli from other neurons. A neuron integrates the excitatory and inhibitory input signals it receives, and generates or fires a series of action potentials in the event that the integration exceeds a threshold potential. A neural firing threshold, for example, may be approximately −55 mV. Action potentials propagate to the neuron's synapses and are then conveyed to other synaptically connected neurons.
Neural activity in the brain can be influenced by neural stimulation, which involves the application of electrical and/or magnetic stimuli to one or more target neural populations within a patient using a waveform generator or other type of device. Various neural functions can thus be promoted or disrupted by applying an electrical current to one or more regions of the brain. As a result, researchers have attempted to treat certain neurological conditions, including ET, using electrical or magnetic stimulation signals to control or affect brain functions.
Deep Brain Stimulation (DBS) is a neural stimulation therapy that has been used as an alternative to drug treatments and ablative surgical therapies. In DBS, one or more electrodes are surgically implanted into the brain proximate to deep brain or subcortical neural structures. For treating ET, an electrode is typically positioned in or proximate to the ventrointermediate nucleus (VIM) of the thalamus. In a typical DBS system, a pulse generator delivers a continuous or essentially continuous electrical stimulation signal having a pulse repetition frequency of approximately 150 Hz to each of two deep brain electrodes. U.S. Pat. No. 5,883,709 discloses one conventional DBS system for treating movement disorders.
Although DBS therapies may significantly reduce ET symptoms, particularly when combined with drug treatments, they are highly invasive procedures. In general, configuring a DBS system to properly function within a patient requires a time consuming, highly invasive surgical procedure for implanting at least one, and possibly two, DBS electrodes. DBS surgical procedures have essentially the same risks as those described above for ablative surgical intervention.
Motor Cortex Stimulation (MCS) is another type of brain stimulation treatment that has been proposed for treating movement disorders, such as ET and Parkinson's disease. MCS involves the application of stimulation signals to the motor cortex of a patient. One MCS system includes a pulse generator connected to a strip electrode that is surgically implanted over a portion of only the motor cortex (precentral gyrus). The use of MCS to treat symptoms associated with Parkinson's Disease is described in Canavero, Sergio, “Extradural Motor Cortex Stimulation for Advanced Parkinson's Disease: Case Report,” Movement Disorders (Vol. 15, No. 1, 2000).
Because MCS involves the application of stimulation signals to surface regions of the brain rather than deep neural structures, electrode implantation procedures for MCS are significantly less invasive and time consuming than those for DBS. As a result, MCS may be a safer and simpler alternative to DBS for treating ET symptoms. Present MCS techniques, however, fail to address or adequately consider a variety of factors that may enhance or optimize the extent to which a patient experiences short term and/or long term relief from ET symptoms.